Color television systems have been developed using several different broadcast and signal processing formats to achieve the successful transmission and reception of color television programming. While substantial differences between systems exist, they all must satisfy the basic objective of combining the picture or luminance information, the color or chrominance information, and sound information together with appropriate display scan synchronizing signals to form an information signal which may be modulated upon a carrier for transmission. At the receiver, the opposite processes must take place in which the several components of the information signal are separated and appropriately processed. In most television broadcast formats such as the NTSC system used within the United States of America and the PAL system used in many European countries, the signal components corresponding to luminance, chrominance and sound are distinguished from each other and separated for individual processing largely on the basis of signal frequencies. For example, in the NTSC system, the available broadcast bandwidth is maintained at 6 megahertz. To conserve channel bandwidth and to transmit up to 4.1 megahertz of video signal, a vestigial sideband format in which the carrier is off center within the 6 megahertz channel bandwidth is used. The chrominance information is modulated upon a chrominance subcarrier separated from the picture carrier by approximately 3.58 megahertz. The sound information is separated from the picture carrier by 4.5 megahertz. To further conserve channel bandwidth, the luminance signal and chrominance signal share a part of the channel bandwidth.
Thus, a low cost receiver is able to select the chrominance, sound and luminance signal portions by using appropriate frequency response networks or filters and thereafter perform individual processing thereon. Unfortunately, the frequency selection process used in most television receivers results in the loss of substantial amounts of information or image content. Perhaps the most notable loss occurs in the video or luminance information which is severely bandwidth limited as a result of the separation of chrominance and sound information. While these losses have been recognized as less than desirable, the basic filtering processes used in most television receivers has made improvement difficult or impractical. Thus, it was recognized early in television system development that effective recovery of the luminance signal components located in the chrominance frequency band would significantly improve picture resolution. Once device type which facilitates separation of luminance from chrominance within the chrominance frequency band is known generally as a comb filter. However, comb filters generally require long delay networks which often prove to be expensive and inaccurate. For example, many receivers have employed an analog glass delay line type comb filter to separate luminance and chrominance information from the shared frequency spectrum. Since glass delay lines generally do not provide accurate delay, factory alignments are generally needed in such receivers to accurately separate luminance and chrominance signals. This, of course, is time consuming and expensive in high volume television production.
One of the approaches contemplated by practitioners in the art seeking to improve the recovery of luminance information from the chrominance frequency band is found in the use of digital signal processing rather than the more pervasive presently used analog signal processing. In a digital processing environment, the separation of chrominance and luminance information within the chrominance band may be carried forward using digital comb filters which provide accurate and cost effective delay networks and therefore accurately and cost effectively separate the luminance and the chrominance signals within the chrominance band. Effective comb filters are more easily realized in the digital environment and extensive memory may be cost effectively achieved. As a result, comb filtering and signal delays are relatively easy to perform in a digital signal environment. With the availability of cost effective comb filter networks which may recover substantially all of the luminance signal components within the chrominance band, an opportunity arises to use the recovered luminance signal components to significantly improve the displayed picture. However, it has been found that the fact that the chrominance band luminance signals are acted upon and respond to some of the chrominance signal processing may degrade the luminance signal. The digital comb filter necessitates an analog-to-digital conversion or "digitization" of chrominance band video signals. In the event the chrominance signal is weak, that is lower in amplitude, the chrominance signal digitization does not utilize the full dynamic range of the analog-to-digital converter. Therefore, the signal to noise ratio of the digitized chrominance band signal is degraded. Therefore, it is important that the proper amplitude of the chrominance band signal be maintained at the input of the analog-to-digital converter. Regulation of the chrominance band signal at the input to the analog-to-digital converter is accomplished by making use of the reference color burst signal which is transmitted during the horizontal blanking interval following the horizontal scan synchronizing pulse. This reference burst is examined by a detector within the automatic chrominance control system and used to control a variable gain amplifier. Thus, if the reference burst decreases, the chrominance band signal gain is increased to compensate and maintain the required chrominance band signal. Conversely, if the reference burst increases, chrominance band signal gain is decreased.
This control of chrominance band signal in response to reference burst is appropriate because the chrominance signal level is related to the reference burst level. However, the luminance signal components are not related to the reference burst and thus changes of luminance signal in response to reference burst changes result in undesired distortion of the luminance signal.
There remains, therefore, a need in the art for a practical system which facilitates the use of digital electronic circuit processing to recover the luminance signal components from the chrominance band while avoiding degrading luminance signal distortion caused by chrominance system processing.
Accordingly, it is a general object of the present invention to provide an improved video processor. It is a more particular object of the present invention to provide an improved video processor which effectively separates the luminance and chrominance signal components within the chrominance signal band with faithful and accurate recovery of each signal component.